A conventional cathode-ray tube comprises a glass bulb having phosphors for red, green and blue inside and an electron gun that emits electron beams inside the glass bulb. This electron gun comprises cathodes, a first control electrode, a second control electrode, and a third control electrode. The cathodes emit a plurality of electron beams having an inline alignment in the horizontal direction. The first control electrode has first electron-beam through holes opposing the respective cathodes. The second control electrode has second electron-beam through holes provided at positions opposing the respective first electron-beam through holes. The third control electrode has third electron-beam through holes provided at positions opposing the respective second electron-beam through holes.
In cathode-ray tubes, the important factors for determining the image quality are the spot diameter of electron beams striking phosphors and the current value of the electron beams in general. That is to say, the smaller the spot diameter of the electron beams is, the more the resolution is improved. The higher the current value of the electron beams is, the higher the brightness of phosphors becomes. Consequently, bright and clear image pictures can be obtained.
However, in the conventional cathode-ray tube described above, when making the spot diameter of the electron beams small and setting the current value of the electron beams high at the same time, the current density of the current obtained from cathodes becomes high. Consequently, the electron emission from the cathodes becomes difficult, thus limiting the picture image to have high brightness. In addition, the driving voltage of the cathodes becomes high, thus causing problems such as a great burden on a driving circuit. On the contrary, when controlling the current density of the current obtained from the cathodes to a predetermined level or less and increasing the current value of the electron beams, the spot diameter of the electron beams becomes larger, thus causing such problems that the high resolution of image pictures is difficult to obtain.